Seismic Perfomance of R/C Frames with Regular and Irregular Strength Vertical Distributions

Abstract eng:
The paper presents the results of a research study concerning the seismic response and design of RC frames with strength discontinuities in elevation. The irregularities are obtained assigning overstrengths either to the beams or to the columns of a “regular frame” (assumed as reference). The “regular frame” is designed according to the Eurocode 8 (EC8) High Ductility Class (DCH) rules. The overstrengths of presumed irregular frames are assigned modifying the reinforcement either of the beams or of the columns at different floors. For all frames the criteria of vertical strength irregularity of many international seismic codes are applied. To this purpose, the storey strengths are computed by two different methods: the first one only takes into account the flexural resistance of columns, while the second one also considers the beam flexural resistance. Non linear static and dynamic analyses are performed: mechanical non linearity is concentrated at the element ends. These analyses are carried out according to EC8 provisions: for non linear static analysis the N2 method is applied; in the case of non linear time-history analyses, seven real earthquakes, selected in order to fit on average the elastic design spectrum are used as input. The seismic response of frames characterised by the assigned overstrength is not very different with respect to the “regular frame” one; this demonstrates that the sensitivity of frames, designed according to High Ductility Class, to overstrength vertical variations is low. Indeed all the frames satisfy the Ultimate Limit State, verified by the application of both the non linear static and the non linear dynamic analysis.

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